Conventional foamed products have been produced using either chemical or physical blowing agents. For example, various chemical blowing agents which are generally low molecular weight organic compounds are mixed into a polymer matrix and decompose when heated to a critical temperature, resulting in the release of a gas (or gases) such as nitrogen, carbon dioxide, or carbon monoxide. Techniques using physical agents include the introduction of a gas as a component of a polymer charge or the introduction of gases under pressure into molten polymer. Such standard foaming processes produce voids or cells within the plastic materials which are relatively large, i.e., on the order of 100 microns, or larger, with the percent of void fraction ranging from 20%-40%, e.g., for structural foams, or 80%-90%, e.g., for insulation foams, of the original material. The number of voids produced per unit volume of polymers is relatively low (on the order of 10.sup.6 cells/cm.sup.3), and there are generally non-uniform cell size distributions throughout the foamed materials.
Extrusion of large cell polymer foam material can be accomplished using a technique known as the Celuka process that incorporates two sequential extruders. A dry mixture of polymer pellets and a nucleating agent are fed into the first extruder. After the mixture has been melted in the first extruder's compression or transition zone, a high pressure physical blowing agent (typically a fluoro- or hydro- carbon gas or fluid) is injected through the barrel of the extruder into the polymer melt. Immediately following such injection, the melt is intensely mixed in the metering zone of the first extruder. The mixture next flows into the second extruder which provides further mixing and meters the flow. At the exit of the extruder is a die (typically an annular or a sheet profile type) which performs the primary shaping of the extrudate. The die is typically constructed such that pressure of the melt drops rapidly across the die lips. At the exit of the die, the expanding profile is surface cooled using a gas jet. As the profile expands, it is drawn over a mandrel or through a die to hold its shape.
A continuous microcellular polymer extrusion process has also been proposed e.g. in U.S. Pat. No. 4,728,559 issued to Hardenbrook et al. on Mar. 1, 1988. The Hardenbrook patent describes a process in which a web of plastic material is impregnated with an inert gas and the gas is diffused out of the web in a controlled manner. The web is reheated at a station external to the extruder to induce foaming, the temperature and duration of the foaming process being controlled to produce the desired cell characteristics. The process is designed to provide for the production of foamed plastic web materials with integral unmodified skins in a continuous manner. A significant limitation of this process is that only thin profiles (on the order of 0.020 inches in thickness) can be microcellular processed due to the limits of thermal cycling in producing thick walled profiles.
In another approach, the batch processing of microcellular plastics provides for pre-saturating a plastic material with a uniform concentration of a gas under pressure and the provision of a suddenly induced thermodynamic instability resulting in the nucleation of a large number of cells. For example, the material is pre-saturated with the gas and maintained under pressure and at a temperature above the glass transition temperature. The material is suddenly exposed to a low pressure to nucleate cells and promote cell growth to a desired size (depending on the final density required) thereby producing a foamed material having microcells. The material is then quickly quenched to maintain the microcellular structure. Such a technique for microcellular materials has been described in U.S. Pat. No. 4,473,665, issued to Martini-Vredensky et al. on Sep. 25, 1984.
Improved techniques have been described in U.S. Pat. No. 5,158,898 issued on Oct. 27, 1992 to Cha et al. A supercritical fluid instead of a gas, is introduced as a foaming agent so as to increase the number of cells nucleated per unit volume of the original material and to produce much smaller cell sizes than those in standard commercial polymer foams. The supermicrocellular process described tends to provide cell sizes that are generally smaller than the flaws that preexist in the polymers so that the densities and the mechanical properties of the materials involved can be controlled without sacrificing the mechanical properties of many polymers.
Microcellular plastics are generally defined as foamed plastics characterized by cell sizes on the order of 10 .mu.m, cell densities on the order of 10.sup.9 cells per cubic centimeter, and specific density reductions in the range of 5 to 95 percent. These cells are smaller than the flaws preexisting within the polymers and, thus, do not compromise the polymers' specific mechanical properties. The result is a lower density material with no decrease in specific strength and a significant increase in the toughness compared to the original polymers. With microcellular plastics, one can use less polymer, thereby substantially reducing material costs while maintaining mechanical properties.
In order to reduce the cell size and to increase the cell density, the above discussed supermicrocellular process was developed for manufacturing foamed plastics having cell sizes of 0.1 to 1.0 micron and cell densities from 10.sup.12 to 10.sup.15 cells per cubic centimeter of the original material, as described in the aforementioned Cha et al. patent. The improved technique provides for saturating a polymer plastic material with a supercritical fluid such as carbon dioxide, which has a higher solubility in the polymers for its supercritical fluid state than for its comparable gaseous state. When the fluid/polymer solution contains a sufficient amount of supercritical fluid at a suitably selected temperature and pressure, the temperature and/or pressure of the fluid/polymer system is rapidly changed to induce a thermodynamic instability and a foamed polymer is produced. The resulting foamed material can achieve a cell density of several hundred trillion voids per cubic centimeter and average void or cell sizes of less than 1.0 micron.
The latter two batch foaming techniques described involve a relatively slow gas-saturating process due to slow gas diffusion. It is desirable to develop an effective technique for saturating polymers with a large amount of gas or supercritical fluid in real time so that the overall cycle time for microcellular processing is significantly reduced and so that microcellular and supermicrocellular plastics can be produced continuously. Furthermore, it is desirable to develop an effective foaming technique so as to produce the desired cell densities in such continuous processes and to do so at a reasonable cost.